Hitherto, a hydraulic drive system for a working machine including a mechanical governor-equipped engine has been proposed as disclosed in, e.g., JP, A 7-83084.
A prior-art system including that type of mechanical governor-equipped engine generally comprises a variable displacement hydraulic pump driven by the engine, a regulator for controlling the displacement of the hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, a pressure sensor for detecting the delivery pressure of the hydraulic pump and outputting a delivery pressure signal, and a controller for receiving the delivery pressure signal outputted from the pressure sensor and outputting, to the regulator, a control signal to control the displacement of the hydraulic pump.
In the prior-art system including the mechanical governor-equipped engine, an engine output characteristic has, in a governor region where a mechanical governor performs control, a drooping characteristic that the engine revolution speed increases as the engine output torque (engine load) reduces. Such a drooping characteristic is produced by the inertia of a flywheel contained in the mechanical governor.
In the case of the working machine being, e.g., a hydraulic excavator, therefore, in a no-load operation after loading earth and sand, etc. in a bucket and then unloading them, the delivery pressure of the hydraulic pump lowers and the engine load reduces, whereby the engine revolution speed increases. This further increases the delivery rate of the hydraulic pump and hence the flow rate of the hydraulic fluid supplied to the hydraulic actuators so that the hydraulic actuators can be operated at relatively high speeds. As a result, the working speed in the no-load operation can be increased and the working efficiency can be improved.
Also, as disclosed in JP, A 10-89111 and JP, A 10-159599, for example, there is conventionally known a hydraulic drive system for a working machine including, instead of the mechanical governor-equipped engine described above, an engine including a fuel injection control unit capable of performing control in a governor region based on an isochronous characteristic or a reverse drooping characteristic (also referred to as an “engine performing isochronous control or reverse drooping control” hereinafter). The isochronous characteristic in engine control means a characteristic that the engine revolution speed is kept constant in the governor region regardless of the magnitude of the engine load, i.e., regardless of a reduction of the engine output torque. The reverse drooping characteristic means a characteristic that the engine revolution speed is reduced as the engine output torque (engine load) decreases.
With that prior-art system, it is possible to prevent the effect due to the inertia of the flywheel as encountered in the mechanical governor, and to realize lower fuel consumption and less noise than those in a working machine including an engine equipped with a mechanical governor.